Assessment Comments
Assessment is based on results of monthly monitoring from 2006 through 2008 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses remain assessed (monitored) as “partially supported.” The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported." Both assessments are based on results of IDNR/UHL ambient water quality monitoring upstream from Ottumwa. The Class C (drinking water) uses are assessed (monitored) as "fully supported" based on results of the IDNR/UHL ambient monitoring. Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this river reach. The source of data for this assessment is the results of monthly monitoring from 2006 through 2008 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003).
EXPLANATION: The Class A1 uses are assessed (monitored) as "partially supported" based on results of monitoring for indicator bacteria (E. coli) at the IDNR/UHL ambient monitoring station on the Des Moines River upstream from Ottumwa. The geometric mean level of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2006 through 2008 (78 orgs/100ml) is below the Iowa Class A1 water quality criterion of 126 orgs/100ml, thus suggesting relatively low levels of bacteria in this river segment The relatively low geometric mean for E. coli for the 2006-08 period is consistent with previous assessments that have shown relatively low levels of indicator bacteria in this river segment. Six of the 22 samples (27%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if the geometric mean of E. coli is less than the applicable state criteria, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). Thus, these data suggest that the Class A1 uses should be assessed as “fully supported. These EPA guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, the monitoring data from 2006-08 suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.”
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of monitoring from the IDNR/UHL ambient station at Chillicothe from 2006 through 2008. Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 34 samples analyzed or for toxic metals in the two samples analyzed during this assessment period.
The Class C (drinking water) uses are assessed (monitored) as "fully supported." During the 2006-2008 assessment period, none of 11 samples collected violated the atrazine MCL of 3 ug/l (3.5 ug/l), and none of 33 samples exceeded the nitrate MCL of 10 mg/l. The average levels of both atrazine (0.11 ug/l) and nitrate (5.9 mg/l) are well below their respective MCLs. Based on IDNR’s assessment methodology, if the average level of nitrate is less than the respective MCLs, the Class C drinking water should be assessed as “fully supported.” Similarly, if the average level of a pesticide is less than the respective MCL, the Class C drinking water should be assessed as “fully supported.”
Fish consumption uses remain not assessed due to lack of recent fish tissue monitoring in this river segment.
